1. Field of the Invention
The present invention relates to a plane light source apparatus, particularly to a plane light source apparatus used as a backlight in an image display apparatus of a liquid crystal display apparatus or the like.
2. Description of the Related Art
FIG. 1 is a disassembled perspective view showing a structure of a plane light source apparatus 1 of a related art and FIG. 2 is a sectional view thereof. The plane light source apparatus 1 is constituted by a light emitting portion 2, a light guide plate 3, a reflecting plate 4 and a prism sheet 5. The light emitting portion 2 is mounted with a plurality of pieces of so-to-speak point light sources 7 of light emitting diodes or the like on a circuit board 6, and arranged opposedly along an end face (light incident end face 8) of the light guide plate 3. The light guide plate 3 is molded by a transparent resin having a large refractive index such as polycarbonate resin or methacrylic resin, and at a lower face (face on a side opposed to a light emitting face 9) of the light guide plate 3, there is formed a diffusing pattern 10 by recessed and projected forming, dot printing of diffusing and reflecting ink or the like. The reflecting plate 4 is formed by, for example, a white resin sheet having high reflectance and both end portions of the reflecting plate 4 are pasted to the lower face of the light guide plate 3 by a double coated tape 11. The prism sheet 5 is arranged with prism patterns 12 having a section in a triangular shape in parallel with each other and placed above the light emitting face 9 of the light guide plate 3.
Further, according to the plane light source apparatus 1, as shown by FIG. 2, light f1 emitted from the light emitting portion 2, is introduced from the light incident end face 8 of the light guide plate 3 to an inner portion of the light guide plate 3. The light f1 confined at inside of the light guide plate 3, progresses at the inner portion of the light guide plate 3 while repeating total reflection by an upper face and a lower face of the guide plate 3. At this occasion, when the light f1 incident on the diffusing pattern 10 and diffused and reflected by the diffusing pattern 10, is incident on the light emitting face 9 of the light guide plate 3 by an angle of incidence smaller than a critical angle of total reflection, as shown by FIG. 2, the light f1 is emitted from the light emitting face 9 to outside of the light guide plate 3. Light emitted skewedly from the light emitting face 9 of the light guide plate 3, passes the prism sheet 5 and is bent to a direction orthogonal to the plane light source apparatus 1 by the prism sheet 5. Further, the light f1 emitted from the lower face of the light guide plate 3 by passing a portion of the lower face of the light guide plate 3 where the diffusing pattern 10 is not present, returns to inside of the light guide plate 3 again by being reflected by the reflecting plate 4 to thereby prevent loss of an amount of light from the lower face of the light guide plate 3.
FIG. 3 is an outline view for explaining operation of the prism sheet 5 in the plane light source apparatus 1. In light diffused and reflected by the diffusing pattern 10, only light incident on the light emitting face 9 by an angle of incidence smaller than the critical angle of total reflection, is emitted from the light emitting face 9 of the light guide plate 3, further, since a diffusing degree of the diffusing pattern 10 is not so much large, the light f1 is emitted from the light emitting face 9 with a comparatively narrow directing angle in a direction inclined from a direction orthogonal to the light emitting face 9. Now, as shown by FIG. 3, considering a case in which the prism sheet 5 is opposed to the upper face of the light guide plate 3 by directing the prism pattern 12 to a side opposed to the light guide plate 3, light skewedly incident on the lower face of the prism sheet 5, is refracted in a direction substantially orthogonal to the prism sheet 5 by refracting operation of the prism pattern 12. As a result, light passing the prism sheet 5, is emitted in the direction substantially orthogonal to the prism sheet 5. FIG. 5 is a graph showing a relationship between an angle of emittance from the prism sheet 5 and a brightness (rate) by constituting a reference by the direction orthogonal to the prism sheet 5, and a bold line curve C1 in FIG. 5 shows a relationship between the angle of emittance and the brightness when the prism pattern 12 is arranged to direct to the side opposed to the light guide plate 3 as shown by FIG. 3.
Further, as shown by FIG. 4, even when the prism sheet 5 is made to be opposed to the upper face of the light guide plate 3 by directing the prism pattern 12 having an apex angle of 60° to the side of the light guide plate 3, light skewedly incident on the prism pattern 12 on the lower face of the prism sheet 5, is refracted in the direction substantially orthogonal to the prism sheet 5 by refracting operation of the prism pattern 12. A relationship between the angle of emittance and the brightness in the case of arranging the prism pattern 12 to direct to the side of the light guide plate 3 in this way, is as shown by a broken line curve C2 of FIG. 5.
Next, FIG. 6 is an outline sectional view showing a plane light source apparatus 13 having other structure. According to the plane light source apparatus 13, light f2 emitted from a light emitting portion and incident on inside of a light guide plate 14 is propagated in the light guide plate 14 while repeating total reflection between an upper face and a lower face of the light guide plate 14. The lower face of the light guide plate 14 is formed with a reflecting pattern 15 in a shape of a triangular prism, and as in light f2 shown in FIG. 6, the light f2 reflected (total reflection or mirror reflection) twice by the lower face of the light guide plate 14, is emitted orthogonally from the upper face (light emitting face 16) of the light guide plate 14.
When a plane light source apparatus is used as a backlight of a liquid crystal display panel, a viewing angle easy to see an image of particularly, a liquid crystal display panel is said to be about 30° (±15°). However, according to the plane light source apparatus as shown by FIGS. 1 through 3 and the plane light source apparatus as shown by FIG. 4, a directing property of light emitted from the prism pattern 12 is wide as in the bold line curve C1 or the broken line curve C2 shown in FIG. 5 and light is emitted in other than the direction orthogonal to the prism pattern 12 and loss of light is considerable.
For example, according to the plane light source apparatus as shown by FIG. 1 through FIG. 3, as shown by FIG. 7, in order to constitute a spreading angle of light passing through the prism sheet 5 by 30°, it is necessary that a spreading angle of light emitted from the light guide plate 3 is set to about 20°. According to the plane light source apparatus disposed with prism sheet 5 as shown in FIG. 4, in order to constitute a spreading angle of light passing through the prism sheet 5 by 30°, it is necessary that also a spreading angle of light emitted from the light guide plate 3 is set to about 30°. However, actually, according to the plane light source apparatus, the directing property is as wide as shown by FIG. 5 and particularly, light emitted to outside of the viewing angle easy to see, constitutes the loss.
Further, according to a liquid crystal display apparatus using the plane light source apparatus placing the prism sheet 5 above the upper face of the light guide plate 3 as shown by FIG. 3 or FIG. 4 and overlapping a liquid crystal display panel thereabove, the prism sheet 5 is disposed right under the liquid crystal display panel and therefore, there is brought about a drawback that a defect having a size of about 0.1 mm of the prism sheet 5 is seen. Further, since the prism sheet 5 is disposed below the liquid crystal display panel, there is a high concern of producing Moire fringes between pixels of the liquid crystal display panel and the prism pattern 12 of the prism sheet 5.
Further, according to the plane light source apparatus 13 having the structure as shown by FIG. 6, as shown by FIG. 9, light f2 leaked from the lower face of the light guide plate 14 can be returned into the light guide plate 14 again by reflecting the light by a reflecting plate 17 laid at the lower face of the light guide plate 14. However, when the light f2 which has returned into the light guide plate 14, passes the light guide plate 14, the light f2 is emitted in a skewed direction from the upper face of the light guide plate 14 and is not emitted in the direction orthogonal to the light guide plate 14 to thereby constitute the loss.
Therefore, according to the plane light source apparatus of the related art, an efficiency of utilizing light emitted from the light emitting portion is not sufficient and a plane light source apparatus having a higher efficiency is requested.